1. Field of the Invention
The present invention relates to an organic-inorganic composite resin composition, an organic-inorganic composite resin material, an optical element, and a stacked diffraction optical element. In particular, the present invention relates to an optical element used for an imaging optical system such as a camera.
2. Description of the Related Art
Conventionally, in a refracting optical system using refraction of light, lenses made of glass materials having different dispersion characteristics are combined so as to reduce chromatic aberration. For instance, in an objective lens of a telescope or the like, a glass material having small dispersion is used for a positive lens, while a glass material having large dispersion is used for a negative lens, and those lenses are combined for correcting chromatic aberration occurring on axis. However, if a structure or the number of lenses is limited, or if glass materials to be used are limited, it may be difficult to achieve sufficient correction of chromatic aberration.
Therefore, it is known that the use of combination of a refractive optical element having a refracting surface and a diffraction optical element having a diffraction grating can suppress chromatic aberration with a small number of lenses.
This utilizes a physical phenomenon that the direction of generating chromatic aberration in a light beam having a reference wavelength is opposite between a diffracting surface and the refracting surface as an optical element. In addition, by changing a period of the diffraction grating formed on the diffraction optical element in a continuous manner, an equivalent characteristic to a characteristic of an aspheric lens can be obtained.
However, one light beam entering the diffraction optical element is split into multiple light beams having different orders by the diffraction action. In this case, diffracted light beams having other orders than the design order may form images at positions different from a position of image formation by the light beam of the design order and become a factor of generation of flare.
U.S. Pat. Nos. 5,847,877 and 6,262,846 disclose that, by optimizing refractive index dispersion of each optical element and a shape of grating formed on an interface of an optical element, high diffraction efficiency can be achieved in a wide wavelength range. By concentrating light beams in a used wavelength region into a predetermined order (hereinafter, referred to as a design order), intensities of diffracted light beams of the other diffraction orders are controlled to be low so as to suppress generation of flare.
Specifically, U.S. Pat. No. 5,847,877 describes use of BMS81 (nd=1.64, νd=60.1, manufactured by OHARA Inc.) and plastic optical material PC (nd=1.58, νd=30.5, manufactured by Teijin Chemicals Ltd.). In addition, U.S. Pat. No. 6,262,846 describes use of COO1 (nd=1.52, νd=50.8, manufactured by DIC Corporation), plastic optical material PC (nd=1.58, νd=30.5, manufactured by Teijin Chemicals Ltd.), BMS81 (nd=1.64, νd=60.1, manufactured by OHARA Inc.), and the like.
Note that, Abbe's number (νd) is calculated by the following equation (1).νd=(nd−1)/(nF−nC)  (1)
Here, nd denotes refractive index on d-line (587.6 nm), nF denotes refractive index on F-line (486.1 nm), and nC denotes refractive index on C-line (656.3 nm).
The inventors of the present invention have studied optical materials that are available in the market or are under research and development as optical materials of the diffraction optical element, and have obtained distribution graphs as illustrated in FIGS. 2A and 2B. FIG. 2A is a graph illustrating a distribution of Abbe's numbers and refractive indexes of general optical materials. FIG. 2B is a graph illustrating a distribution of Abbe's numbers and anomalous dispersion characteristics of general optical materials. Materials of stacked diffraction optical elements described in U.S. Pat. Nos. 5,847,877 and 6,262,846 are also included in the distributions of FIGS. 2A and 2B.
In addition, U.S. Pat. No. 5,847,877 also discloses use of combination of a diffraction optical element made of a material having relatively low refractive index dispersion and a diffraction optical element made of a material having high refractive index dispersion, in order to obtain a structure having high diffraction efficiency in a wide wavelength range.
In other words, as a difference of the refractive index dispersion becomes larger between a material having high refractive index dispersion and a material having low refractive index dispersion, the diffraction efficiency of the constituted optical element becomes higher and hence a field angle of the optical element becomes wider. Therefore, in order to correct the chromatic aberration with high accuracy, it is required to use a material having higher refractive index dispersion (small Abbe's number) and lower refractive index dispersion (large Abbe's number).
U.S. Pat. No. 7,031,078 discloses an optical material having a relationship between the refractive index (nd) and the Abbe's number (νd) satisfying nd>−6.667×10−3νd+1.70, and a relationship between anomalous dispersion characteristic (θg,F) of the refractive index and the Abbe's number (νd) satisfying θg,F-≦−2νd×10−3+0.59. By satisfying these equations, the diffraction efficiency can be improved in the entire visible light region.
Note that, the anomalous dispersion characteristic (θg,F) of the refractive index is calculated by the following equation (2).θg,F=(ng−nF)/(nF−nC)  (2)
Here, ng denotes a refractive index on g-line (435.8 nm), nd denotes a refractive index on d-line (587.6 nm), nF denotes a refractive index on F-line (486.1 nm), and nC denotes a refractive index on C-line (656.3 nm).
In the optical material disclosed in U.S. Pat. No. 7,031,078, as a transparent conductive metal oxide having high refractive index dispersion and low anomalous dispersion characteristic, ITO, ATO, or SnO2 is exemplified.
U.S. Pat. No. 7,663,803 and Japanese Patent Application Laid-Open No. 2009-197217 specifically disclose use of combination of a diffraction optical element made of a material containing metal oxide fine particles such as ITO as the material having high refractive index dispersion and a diffraction optical element made of a material containing metal oxide fine particles such as ZrO2 as the material having low refractive index dispersion.
A typical general structure of the stacked diffraction optical element disclosed in U.S. Pat. No. 7,663,803 is described with reference to FIG. 3. FIG. 3 is a schematic diagram of a stacked diffraction optical element 201. The upper part of FIG. 3 illustrates a top view, and the lower part of FIG. 3 illustrates a cross sectional view. This stacked diffraction optical element has a structure including a transparent substrate layer 202 made of glass or plastic material, on which a layer 203 having a diffraction grating shape and high-refractive index low-dispersion characteristic, and a layer 204 having low-refractive index high-dispersion characteristic are stacked without space between them. Note that, the order of stacking the layer 203 having high-refractive index low-dispersion characteristic and the layer 204 having low-refractive index high-dispersion characteristic may be reversed. In addition, each side of the transparent substrate layer 202 may be a flat surface, a spherical shape, or an aspheric shape. In addition, both the layer 203 having high-refractive index low-dispersion characteristic and the layer 204 having low-refractive index high-dispersion characteristic may be sandwiched between transparent substrate layers.
As for a structure of the stacked diffraction optical element, if the stacked diffraction optical element is used for various lens systems, the height of the grating must be lower. For instance, if the stacked diffraction optical element is used for a wide-angle lens, components of light entering the grating obliquely increase because of the wide field angle. In other words, as the height of the grating is higher, a ratio of blocking of the incident light caused by the grating wall surface increases, and hence an amount of flare increases.
In order to reduce the height of the grating, refractive index characteristics, in addition to refractive index dispersion characteristics, of the materials forming the two diffraction optical elements have a large influence. As a difference of refractive index between two diffraction optical elements is larger, the height of the grating can be designed lower. Thus, an amount of generated flare of the grating wall surface depending on the height of the grating can be reduced.
The diffraction optical element formed by using the optical material described in U.S. Pat. No. 7,663,803 has high diffraction efficiency of 99% or higher in the visible light region, and the height of the grating is 7.3 μm or more. As a result, the reduction of flare depending on the height of the grating is not sufficient.
In addition, as to the material described in U.S. Pat. No. 7,663,803, in order to further reduce the height of the grating, it is conceivable that increasing amounts of'fine particles causes the refractive index dispersion characteristic to change. However, if additive amounts of fine particles increase, it becomes difficult to produce the material because of a problem of viscosity. In addition, it is expected that use in an optical system becomes difficult because of an increase of dispersion. In addition, because a dispersant is relatively increased, it becomes difficult to obtain desired optical characteristics.
Therefore, in the conventional technologies, no material that can be designed to reduce the height of the grating while maintaining high diffraction efficiency in a visible light region has been found.